1. Field of the Invention
The invention relates to a secondary air supply system and secondary air supply method for an internal combustion engine, which supply air into exhaust gas from the internal combustion engine.
2. Description of Related Art
A catalyst is arranged in an exhaust passage of an internal combustion engine, and toxic substances of exhaust gas are purified by the catalyst through oxidation-reduction reaction. In order to activate the catalyst so as to purify exhaust gas, the temperature of the catalyst needs to be higher than or equal to its activating temperature. Therefore, in order to quickly ensure exhaust gas purification performance after a start-up of an engine, the temperature of the catalyst needs to be early increased.
Then, in an art, there is suggested an internal combustion engine that includes a secondary air supply system as described in Japanese Patent Application Publication No. 2007-024034 (JP 2007-024034 A) to thereby early increase the temperature of the catalyst. The secondary air supply system is formed such that a secondary air supply passage in which an air pump that pressurizes air and discharges the pressurized air and an air switching valve that opens to permit or closes to prohibit introduction of air, discharged from the air pump, into exhaust gas are arranged is connected to the upstream side of a catalyst in an exhaust passage. In an internal combustion engine that includes such a secondary air supply system, secondary air is supplied into exhaust gas and the air-fuel ratio of burned air-fuel mixture is enriched to cause unburned fuel to be burned on the catalyst to thereby make it possible to early increase the temperature of the catalyst.
Note that the secondary air supply system described in JP 2007-024034 A is formed such that a secondary air supply passage in which an air pump and an air switching valve are provided is connected one by one to each of two exhaust pipes in which catalysts are respectively arranged. In such a secondary air supply system that includes two secondary air supply units, as the two air pumps simultaneously start operation, a voltage drop of a battery may become excessively large because of large inrush currents of the air pumps. Therefore, in this secondary air supply system, a time difference is provided between the timings at which the two air pumps start operation to thereby suppress a voltage drop of the battery.
Incidentally, in the thus configured secondary air supply system, at the time at which the pressure of supplying secondary air sufficiently increases after a lapse of a predetermined period of time from the start of operation of the air pumps, the air switching valves are opened. Therefore, when a time difference is provided between the operation start timings of the two air pumps, a time difference also occurs between the timings at which the two air switching valves are opened. When the air switching valve is opened, operation sound occurs because of a variation in pressure in the valve. Therefore, as a deviation occurs between the timings at which the two air switching valves open, the above described sound successively occurs twice, so the operation sound of each air switching valve becomes a tone that causes a passenger to experience a feeling of strangeness.